And I'm Audie Cornish. The crisis in Ukraine may mark a turning point for the North Atlantic Treaty Organization. The military alliance between the United States and its European partners will be a key focus for President Obama next week. He visits NATO headquarters in Brussels on Wednesday. NPR's Ari Shapiro will be on that trip.

ARI SHAPIRO, BYLINE: For the last decade or so, NATO's single biggest focus was Afghanistan. That combat mission ends this year. And so the military alliance had been trying to figure out what next. Judy Dempsey is a NATO expert with Carnegie Europe.

JUDY DEMPSEY: NATO comes home and has to decide, oh, now what do we do? But it's clear now that there is a threat facing the alliance and that threat is called Russia.

SHAPIRO: This week at the Brookings Institution, NATO Secretary General Anders Fogh Rasmussen said, we live in a different world than we did less than a month ago.

ANDERS FOGH RASMUSSEN: This is the gravest threat to European security and stability since the end of the Cold War.

SHAPIRO: He called President Vladimir Putin a global bully and cancelled some planned meetings between NATO and Russia. The chilly attitude goes both ways. When Putin spoke in Moscow this week, he specifically denounced NATO's expansion up to Russia's edges.

PRESIDENT VLADIMIR PUTIN: (Through translator) We were cheated. We were deceived. Some decisions were taken behind our back and the same was with NATO's extension to the east, the same was deployment of the military infrastructure near our borders. We were given the same mantra. They said, well, it's none of your business.

SHAPIRO: Ukraine is not a NATO member but other eastern European countries that lie very close to Russia are, like Poland, Latvia and Estonia. Dempsey of Carnegie Europe says those countries are understandably nervous right now.

DEMPSEY: NATO has to reassure its east European allies that Article 5 there's this collective alliance that if one country is threatened or attacked, the rest of the alliance will come to its assistance. The east Europeans want this reassurance and they're getting it.

SHAPIRO: Getting it in a steady stream. Yesterday President Obama spoke on the White House south lawn before he left for Florida.

PRESIDENT BARACK OBAMA: America's support for our NATO allies is unwavering. We're bound together by our profound Article 5 commitment to defend one another and by a set of shared values that so many generations sacrificed for.

SHAPIRO: It was the second time this week Obama made that commitment. And Vice President Joe Biden flew all the way to Europe this week just to give Poland and Lithuania this assurance in person.

VICE PRESIDENT JOE BIDEN: The reason I traveled to the Baltics was to reaffirm our mutual commitment to collective defense.

SHAPIRO: NATO's Anders Fogh Rasmussen says the military alliance is offering more than just words. He spelled out concrete steps that are increasing NATO's military readiness.

RASMUSSEN: They include more assets for our Baltic air policing mission, surveillance flights over Poland and Romania and heightened awareness.

SHAPIRO: This is a big change. Until now the Obama administration always talked about a pivot to Asia, leaving Europe feeling like an afterthought. Other countries in the alliance were not putting a lot of money into NATO defense spending. Now NATO is suddenly a key player in a global crisis right in its backyard. Cadre Leek is a NATO expert at the European Council on Foreign Relations.

CADRE LEEK: It was said ten years ago or so that NATO has to go out of area or out of business. And not many people say that it's back into area and back into business.

SHAPIRO: NATO also benefits from the fact that this crisis is far less controversial than the wars in Iraq and Afghanistan. Members of the alliance seem more unified on Russia than they've been in a long time. Ari Shapiro, NPR News, London. Transcript provided by NPR, Copyright NPR.